The present invention relates to a hydraulic machine such as a press machine, an injection molding machine, and a die-cast machine in which hydraulic oil sent out from a hydraulic pump is fed through a hydraulic circuit to a cylinder chamber of a reciprocation mechanism to actuate the reciprocation mechanism for reciprocation. In particular, the invention relates to a hydraulic machine in which hydraulic oil is fed to some of a plurality of cylinder chambers in a reciprocation mechanism to reciprocate a die and the like, so as to apply pressure to a workpiece for the bending.
In a conventional typical press brake, a lower die having a V-shaped groove is mounted on a table, and an upper die is attached via a holder to the lower end of a ram so that the both dies are opposed to each other. On the lower die is supported a workpiece plate form, to which the upper die is moved up and down together with the ram. In descending, the upper die applies a pressure force onto the workpiece to press the workpiece into the aforesaid V-shaped groove, thereby bending the workpiece at a prescribed bending angle.
The bending angle of workpiece depends on the lowering terminal position of the upper die. The lowering terminal position, i.e., the stroke of the upper die can be changed to set the bending angle of the workpiece to a desired value.
Press brakes for use in such bending with the aim of realizing both the speed-up of the bending operation and the improvement in precision of the bending at the same time include that of a hydraulic drive system in which a hydraulic cylinder is used as the drive unit of a ram, and that of a motor drive system in which a servomotor is used as the drive unit of the ram.
In the press brake of a hydraulic drive system, servo valves and relief valves are interposed in a hydraulic circuit for feeding hydraulic oil from the hydraulic pump to the hydraulic cylinder. The aforesaid servo valves are actuated to control the feeding route and the feeding amount of the hydraulic oil to the hydraulic cylinder, thereby realizing the up-and-down movements of the upper die and carrying out the speed control and the position control of the upper die as well.
Meanwhile, in the press brake of a motor drive system, a feed screw for moving the ram up and down is connected to a servomotor so as to provide a certain speed reduction ratio. The servomotor can be controlled in number and direction of revolutions to realize the up-and-down movements of the upper die and carry out the speed control and the position control of the upper die as well.
In recent years, press brakes are strongly desired for higher performances. In this view, approaches have been made by the inventor of the present invention in order to realize the further speed-up of the bending operation and the further improvement in precision of the bending.
On press brakes of the hydraulic drive system, studies were made as to the use of a double cylinder which comprises a first cylinder chamber of large cross-sectional area for actuating the upper die at a low speed and a second cylinder chamber of small cross-sectional area for actuating the upper die at a high speed.
On press brakes of the motor drive system, studies were made as to the incorporation of two types of speed reducing mechanisms and a clutch mechanism into the drive mechanism. Here, the aims of the two types of speed reducing mechanisms are to set the speed (i.e., number of revolutions) of the aforesaid feed screw at two levels of high and low speeds, respectively, and the clutch mechanism is to switch the speed reducing mechanisms.
Each of the approaches has brought some degree of results in improving the performance of the press brakes. However, in the press brake of the hydraulic drive system, the hydraulic circuit needs to be improved in responsivity to secure the positioning accuracy of the ram and to shorten the positioning time. This requires the maintenance of a large differential pressure across the upstream side of the servo valve into which the hydraulic oil inflows and the downstream side of the servo valve from which the hydraulic oil outflows, resulting in a greater energy loss. The energy loss is also generated when an excess of the hydraulic oil at the upstream side is returned via the relief valve to a tank as well as when the hydraulic oil is sent through the servo valve. The aforesaid energy loss is converted into thermal energy producing a rise in temperature and a change in viscosity of the hydraulic oil, leading to a problem of adversely affecting the servo control.
Meanwhile, in the press brake of the motor drive system, the two types of speed reducing mechanisms and the clutch mechanism need to be incorporated into one drive mechanism. This complicates the configuration of the drive mechanism to a large extent, giving rise to a problem in increased size and cost of the drive mechanism.